Clock Conditioning Circuits in IGLOO and ProASIC3 Devices
v1.1
4 - 41
Recommended Board-Level Considerations
The power to the PLL core is supplied by VCCPLA/B/C/D/E/F (VCCPLx), and the associated ground
connections are supplied by VCOMPLA/B/C/D/E/F (VCOMPLx). When the PLLs are not used, the Actel
Designer place-and-route tool automatically disables the unused PLLs to lower power
consumption. The user should tie unused VCCPLx and VCOMPLx pins to ground. Optionally, the PLL
can be turned on/off during normal device operation via the POWERDOWN port (see
Table 4-3 onPLL Power Supply Decoupling Scheme
The PLL core is designed to tolerate noise levels on the PLL power supply as specified in the
datasheets. When operated within the noise limits, the PLL will meet the output peak-to-peak
jitter specifications specified in the datasheets. User applications should always ensure the PLL
power supply is powered from a noise-free or low-noise power source.
However, in situations where the PLL power supply noise level is higher than the tolerable limits,
various decoupling schemes can be designed to suppress noise to the PLL power supply. An
example is provided in Figure 4-34. The VCCPLx and VCOMPLx pins correspond to the PLL analog power supply and ground.
Actel strongly recommends that two ceramic capacitors (10 nF in parallel with 100 nF) be placed
close to the power pins (less than 1 inch away). A third generic 10 F electrolytic capacitor is
recommended for low-frequency noise and should be placed farther away due to its large physical
size. Actel recommends that a 6.8 H inductor be placed between the supply source and the
capacitors to filter out any low-/medium- and high-frequency noise. In addition, the PCB layers
should be controlled so the VCCPLx and VCOMPLx planes have the minimum separation possible, thus
generating a good-quality RF capacitor.
Recommended 100 nF capacitor:
Producer BC Components, type X7R, 100 nF, 16 V
BC Components part number: 0603B104K160BT
Digi-Key part number: BC1254CT-ND
Digi-Key part number: BC1254TR-ND
Recommended 10 nF capacitor:
Surface-mount ceramic capacitor
Producer BC Components, type X7R, 10 nF, 50 V
BC Components part number: 0603B103K500BT
Digi-Key part number: BC1252CT-ND
Digi-Key part number: BC1252TR-ND
Figure 4-34 Decoupling Scheme for One PLL (should be replicated for each PLL used)
IGLOO/e or
ProASIC3/E
Device
Power
Supply
VCCPLx
VCOMPLx
10 nF
100 nF
10 F